I love the food at NeptuneOyster and have celebrated a birthday of my own there, but it really depends on what you mean by "atmosphere". It's a very small place that is very popular, so it does get pretty crowded/tight. If you were hoping for dimly lit and quiet....this isn't it.

Neptune Oysters essentially is only a bar--a long, attractive wooden bar with a half dozen-tables against the opposite end of the room. Cramped, in a cozy continental Europe sort of way.

They serve up excellent oysters and lobster rolls, as well as fantastic second courses including great pastas and crudos, in a lovely European-esque bar.

I'm not sure, however, that Neptune Oysters is what you are seeking. It is more akin to your favorite place once you move here. Plus, it gets crowded, they don't take reservations, and the best seats are at the bar. I am not sure that adds up to the night you are looking for.

In lieu, I'd recommend Craigie on Main. Excellent, and they are great with seafood. It will also be crowded, but they have space and take reservations. The atmosphere is lively, with a great bar and open kitchen. Do make a reservation.

You could always start the night early at Neptune with a plate of raw oysters and a crisp glass of Chablis, then taxi over to Craigie for main course and dessert!

What about Oceanaire? It's right at Government Center & is "seafood centric". Maybe Joanie can give some insight / menu recommendations there since I haven't been there yet. If it's a nice night, you could always stroll thru Faneuil Hall (it's very pretty with the trees lit) & head to the North End for desert - Cafe Vittoria has great coffee & pastries.

Oceanaire does indeed have some great seafood. Fish choices change daily depending on what they get in. I had Hawaiian big eye tuna that was simply grilled in olive oil that was amazing, and also pricey $38 plus $6 for a side of cole slaw. It was worth it. I do feel, however, that Neptune, despite it's small space offers an atmosphere that IMHO feels special. Depending on the time the OP would like to dine it may still be doable. I find if you dine early (5:00) you generally can get a seat at the bar, which I actually prefer. The table space feels too tight and the bar space seems just more intimate.

I've only been to Oceanaire twice and had crab cakes both times, plus the awesome hash browns. 2nd time split a bunch of apps so also had a wonderful spinach/bacon salad and good chili shrimp. The room is amazing so it could work for this occasion but as noted, pricey entrees. I think Neptune is nice and a fun atmosphere if you can deal with the cramped quarters and no reservations, would serve the purpose if the OP knows they've got items they want to eat. Some people like Pigalle which would be a short walk from your place on Charles St. Or Hamersley's in the south end.

Craigie, while perhaps better known for meat, really shines with their seafood, and I have been more and more going the fish route there rather than meat. Rendezvous is also quite good for fish/seafood.

If you want to eat in Cambridge, I think Craigie on Main is a great choice. Rendezvous is really nice too. But if I had to pick one spot in Cambridge that is truly my favorite, it'd be Hungry Mother. It's a casual restaurant with amazingly delicious food --appalachian/southern focus cooked in the classic french style. It can be cozy and romantic and though it's not fancy by any means, I always feel like it's a special night out when we dine there. I am not a huge meat eater either and you can find excellent seafood/vegetarian dishes at any of these spots. Enjoy!

So far, plans are Neptune Oyster at 5:00 on Friday then off to The Donkey Show in Cambridge after which looks fun! I might need some lunch or cocktail ideas. Lunch will be light of Friday, more of an event on Saturday. Sounds crazy but I have always wanted to try Pizzaria Regina on the North end. How about any must see gourmet shops or great shopping areas? What is the cheese shop, Formaggio? Thanks all!